My point about modifier keys is that they work differently on virtual keyboards than on real keyboards. On virtual keyboards, it is the keyboard that assigns a new character to the <modifier><letter> combination. This means that xmodmap is less useful for customizing virtual keyboards than physical keyboards.
The iLiad keyboard is a replacement for
matchbox-keyboard which I understand used to be provided with the iLiad. As the attached image shows, it has control keys.
xvkbd looks even better, but matchbox-keyboard may be good enough.